Method for cambering glass sheets by suction

ABSTRACT

A method and device for bending superposed sheets of glass. The sheets are picked up by a top form furnished with a suction creating an upward airflow blowing over the rim of the sheets, the suction being sufficient to lift and hold the superposed sheets against the top form, then the sheets are pressed between the top form and a full surface solid concave bottom form furnished with openings, the pressing beginning conducted while the suction is not yet finished or is finishing, then the superposed sheets are formed, by suction of the main face of the bottom sheet through the openings of the bottom concave mold, the forming by suction beginning while the pressing is not yet finished, and then the sheets are cooled. Windshields free of optical defects may thus be produced.

BACKGROUND

1. Field

The invention relates to a method of bending glazing units by which theglass is sucked by openings passing through a solid concave mold to takethe form of said concave mold. The method according to the invention ismore particularly suited to a rapid industrial production process,leading to glazing units free of optical defects.

The invention relates most particularly to the simultaneous bending ofsuperposed sheets of glass (usually two superposed sheets of glass) thatare required to be assembled later into a laminated glazing unit,particularly of the type of sheets intended to serve as windshields ofmotor vehicles. The sheets are assembled in a manner known to thoseskilled in the art by inserting between the sheets of glass a layer ofpolymer, usually of the polyvinylbutyral type.

2. Description of the Related Art

EP0363097 teaches the suction of a single sheet through a bottom concavemold. After bending, the sheet is separated from the bottom mold by aframe surrounding the concave form and rising to carry the sheet withit. To be able to carry the sheet upward, the latter must protrudebeyond all the edges of the concave mold. Such a protrusion is notsatisfactory for bending the edges, which are not always preciselyformed during the bending. The result of this is a risk of bendingdefects at the edges of the sheet. Furthermore, this document gives noinformation relating to the bending of several superposed sheets.

U.S. Pat. No. 3,778,244 teaches a top form fitted with a suction skirtplaced above a bottom mold furnished with suction orifices through itssolid full surface. These two bending forms work one after the otherwithout carrying out any pressing of the glass, since, when a sheet hasbeen partially bent against the top form, this sheet is allowed simplyto fall on the bottom form (col 5 lines 3-6).

WO02064519 teaches the bending of superposed sheets through a bottomconcave mold. A top oddside mold may squeeze the edges of the two sheetstogether at the time of suction by the concave form. Here also, it is aframe surrounding the bottom form which, when rising, picks up thesheets after bending. Consequently, the sheets must also protrude fromthe bottom form. Furthermore, even though this method represents notableprogress, the desire is to be able to accelerate it in the context of anindustrial usage. To be able to accelerate it, it is possible to work onthe value of the vacuum used during the suction. The applicant hashowever observed that it was appropriate not to push the vacuum beyond avacuum pressure of approximately 100 mbar (the difference betweenatmospheric pressure and the pressure in the bottom mold), because thatcould lead to optical defects visible to the naked eye, of the distortedvision type. It seems that this defect originates in fact from aclearance being created between the various sheets during the suction.This clearance may even reach 1 mm in the central region. The opticaldistortions that result therefrom may reach 90 mdpt (millidioptres).

SUMMARY

The invention resolves the aforementioned problems. According to theinvention, upward suction means are used, particularly of the suctionskirt type, to pick up the sheets of glass, before the suction bendingstep. It has been discovered that the use of this type of means ofsupport procured a double advantage: 1) on the one hand, it is no longernecessary for the sheets to protrude beyond the bottom concave mold, 2)on the other hand, and unexpectedly, if the suction/holding step (upwardsuction) is correctly combined with the suction/bending step (downwardsuction) through the bottom mold, it is possible to push the vacuum inthe bottom concave mold beyond a vacuum pressure of 100 mbar without,for all that, causing optical defect problems. As a result, theinvention allows superposed sheets to be bent at very rapid productionrates.

Without this explanation causing a limitation to the scope of thepresent invention, it seems that the upward suction removes asubstantial portion of the air between the superposed sheets andcontributes to a better combination of the superposed sheets. Theconsequence of this is that the sheets are all more accurately broughttoward the concave bottom form during the suction/bending step on thebottom mold. Specifically, it seems that the presence of residual airbetween the sheets could prevent a sheet that is not in direct contactwith the bottom mold from following the bottom sheet during the suctionbending step.

Thus, the invention relates in particular to a method of formingsuperposed sheets of glass (usually two superposed sheets of glass)brought to their forming temperature, comprising:

-   -   a suction/holding step comprising the sheets being picked up by        a top form furnished with suction means creating an upward        airflow blowing over the rim (edge) of the sheets, said suction        being sufficient to lift and hold the superposed sheets against        said top form, then    -   a pressing step comprising the pressing of the sheets between        the top form and a full surface solid concave bottom form        furnished with openings (holes), said pressing beginning while        the suction of the preceding step is not yet finished or is        finishing, then    -   a suction/bending step comprising the forming of the superposed        sheets, by suction of the main face of the bottom sheet through        the openings of the bottom concave mold, said suction forming        beginning while the pressing of the preceding step is not yet        finished, then    -   a cooling step comprising the cooling of the sheets.

According to the invention, it has been discovered that the suctionexerted during the suction/holding step serves not only to hold thesheets in order to place them in the appropriate position above thebottom bending mold, but it also draws the air from between the sheets.That is why it is preferred according to the invention to begin thepressing step while the upward suction is still operating or is in theprocess of finishing, because the pressing step procures an intimatecontact between the sheets and ensures the seal between them, at a timewhen the air has been drawn from between the sheets. In practice, theupward suction may be stopped as soon as the pressing step begins, sothat stopping the upward suction may be virtually simultaneous withbeginning the pressing step. Specifically, no elastic effect intervenesto separate the sheets. This pressing step therefore prevents air fromreturning between the sheets. The sheets are then ready to be bent bysuction of the bottom sheet. This (downward) suction by the bottom moldbegins although the pressing step is not yet finished. The absence ofair between the sheets means that all the sheets correctly follow thebottom sheet while it is being formed. It was noted that it was possibleto exert a vacuum pressure of 350 mbar at the suction/bending step whichresulted in a maximum clearance between the sheets of 0.5 mm, and anoptical distortion of less than 70 mdpt, these values being capable ofvarying depending on the geometry and the complexity of the bendingforms sought. It is not necessary to continue the pressing throughoutthe bending by suction. Specifically, once the bending by suction hasbegun and once the viscoelastic stresses are relaxed, the pressing canbe released because the superposed sheets remain well pressed together.Preferably, the suction of the suction/bending step is thereforeexecuted for a sufficient period for the required shape to be obtainedand for the viscoelastic stresses after obtaining this final shape to berelaxed.

The top form picking up the sheets may be a full surface solid form orpreferably a frame. “Frame” means a strip of an appropriate material(usually metal) offering at the periphery of the sheet placed in the topposition a contact surface usually from 0.5 mm to 10 cm wide.“Periphery” means the border zone, of annular shape, of a main face ofthe sheet situated at less than 15 cm from all the edges.

The pressure exerted during the pressing step may be very light.Specifically, it is usually sufficient to push the top sheet a little(while the upward suction is still being exerted or is in the process ofstopping) so that the two sheets are more intimately juxtaposed.Preferably, this pressure is exerted in the peripheral zone of thesheets. Preferably, the pressure is exerted so as to prevent air passingbetween the sheets.

The bottom bending mold is concave and its concavity correspondssubstantially to the desired final shape of the glazing unit. Thisbottom mold is solid and comprises, at its surface, openings allowingthe suction of the sheet that is placed in the bottom position and withwhich it is in contact. This bottom mold is preferably at least as largeas the bottom sheet, so that the superposed sheets do not protrudebeyond the mold at any location.

Once the suction through the bottom mold has begun, it is possible tostop the pressure by separating the top form and the bottom mold fromone another. At this moment, there is no more upward suction. Thedownward suction through the bottom mold usually lasts from 1 s to 40 s.

The suction forces exerted on the sheets on the one hand during thesuction/holding step (upward) and on the other hand during thesuction/bending step (downward) are therefore different. Specifically,the first case involves creating a vacuum pressure at the border of thesheets so as to lift the sheets upward and also suck the air out frombetween the sheets (high airflow, for example of the order of 25000 m³of air per hour for a 1.8 m² windshield), and the second case involvespressing the sheet in the bottom position against the bottom mold (largevacuum pressure and low airflow). The upward suction therefore requiresthe creation of a strong airflow blowing over the rim of the sheets,whereas the downward suction (for bending) requires the air to be drawnoff through openings distributed substantially over the whole surface ofthe bottom mold. Usually, the upward suction of the suction/holding stepis generated through a skirt surrounding the top form. To be better ableto pick up and raise the sheets, the skirt surrounding the top form maycome into contact with the support on which the sheets rest. If thissupport is a frame, its central part allows the air to pass beneath thesheets of glass which are then lifted without difficulty. If the supportis not a frame, at least one orifice can be provided in the supportbeneath the sheets so that the air sucked through the skirt more easilyraises the sheets.

To be picked up at the suction/holding step, the superposed sheets maybe flat or already pre-bent before said step and have a temperatureallowing them to be bent (usually between 560 and 610° C.). They arepreferably pre-bent. Usually, this pre-bending has been carried out bygravity collapsing on a pre-bending support, usually of the frame orskeleton type. When the bends to be applied during the pre-bending stepare relatively major, an articulated skeleton or any type of skeletonsubjecting the sheets to several shapes successively may be used. Inparticular, these may be the skeletons described in EP 448447, EP 705798or application PCT/FR2004/050198. Any pre-bending step gives the sheetsthe shape of a rough, part way between the flat shape and the desiredfinal shape. This pre-bending may also give the periphery of the glazingunit its final shape, whereas the central part is only roughed. Theexistence of a pre-bending step is preferred when the final shape hasrelatively accentuated curves, particularly when the final shape hascurves in directions orthogonal to one another (double bending). Motorvehicle manufacturers are increasingly making use of this type ofcomplex shape having considerable double bending. This pre-bending isusually achieved by gravity, the superposed sheets being placed on anappropriate support which travels through a tunnel oven toward the mainbending zone comprising the top form and the bottom bending mold. Thetunnel oven is used both for giving the sheets the bending temperatureand for carrying out the pre-bending. The pre-bending supports travelthrough the tunnel oven for example by being mounted on wheeled trolleysplaced on rails. The pre-bending supports pass beneath the top form,which then picks up the superposed sheets thanks to the upward suctionas already explained.

As already mentioned, the pre-bending support, particularly of theskeleton type, may offer, as a surface of contact with the glass, ashape that changes as it moves. This change may take place during themovement toward the position beneath the top form, while the supportcarries the superposed sheets. In particular, the contact surface of thepre-bending support may take at the periphery the final shape desiredfor the glass, even before being picked up by the top form. This doesnot mean that the glass already takes the desired final shape beforebeing picked up by the top form, because the collapsing of the glass atthe periphery may not be finished at that time and furthermore, even ifthe periphery of the glass touches the pre-bending support everywhere,the central zone has not, at any rate, taken the final desired shape. Ifthe pre-bending support has the desired final peripheral shape for thesheets when picked up by the top form, this support may also pick up thesheets after the final bending in order to take them to the cooling zonewithout it being necessary to change its shape again. It is alsopossible for the pre-bending support to keep a rough peripheral shapeuntil picked up by the top form, then, when it is no longer supportingthe sheets, take the final peripheral shape desired for the sheets.Specifically, if this support must pick up the sheets after the finalmolding by the bottom mold, in order to take the sheets to the coolingzone, it is preferable for this support to have a shape that correspondswell to the final shape desired for the sheets.

Before any heating of the sheets, it is possible to place between thesheets superposed in an assembly a powder preventing the various sheetsfrom sticking together during the bending process. This powder (possiblybeing silica for example) is placed in a manner known to those skilledin the art.

The contact surface of the top form may correspond to that of the topsheet when it is ready to be picked up, that is to say it may be flat ifthe sheets arrive flat beneath it, or have a rough shape correspondingto that given to the sheets by any pre-bending process, or have, atleast at the periphery, the desired final shape. Preferably, the topform has at the periphery the desired final shape. In any case, the topform has no curves less accentuated than those of the sheets that itmust pick up.

When the bending is finished and the sheets are resting on the bottombending mold, the sheets may be cooled. To do this, it is preferable toseparate the sheets from the bottom mold in order to subsequently placethem on a support called a cooling support. The sheets may for examplebe separated from the bottom mold by upward suction in a manner similarto that used during the suction/holding step. It is even possible to usethe same top form to carry out the suction/holding step and theseparation from the bottom mold. However it is also possible to use asecond top form (which may be called FS2 in the context of the presentapplication) furnished with the necessary suction means (particularly ofthe skirt type), surrounding it, to carry out this separation operation.After separation from the bottom mold by picking up with a top form, thelatter then releases the sheets on top of a cooling support whichcarries the sheets toward the appropriate cooling zone. When a secondtop form is used, the latter advantageously has a contact shape with thetop sheet corresponding to the final shape of the glazing unit. In allcases, the cooling support advantageously has a contact shape facing thesheet placed in the bottom position corresponding to the final shape ofthe glazing unit. Usually the cooling process is close to naturalcooling, of the type used on the sheets intended to incorporate alaminated glazing unit for a motor vehicle windshield.

Thus, according to the invention, the bent superposed sheets may beseparated from the bottom concave mold after the suction/bending step,by being picked up with the aid of a top form furnished with suctionmeans creating an upward airflow blowing over the rim of the sheets,said suction being sufficient to lift and hold the superposed sheetsagainst said top form, said top form then letting the bent superposedsheets rest on a support taking them toward a cooling zone for thecompletion of the cooling step.

In particular, the top form separating the bent sheets from the bottommold may be a second form (FS2) distinct (but possibly having the sameshape) from the first top form (FS1) picking up the sheets at thesuction/holding step, the sheets possibly being moved from a positionbeneath the form FS1 to a position beneath the form FS2 by the movementof the bottom mold supporting them.

The use of two top forms makes it possible to accelerate the rates ofthe process relative to the variant with a single top form.Specifically, when the second form (FS2) lets a set of bent superposedsheets rest on a support that is to take it to a cooling zone for thecompletion of the cooling step, the bending of another set of superposedsheets may already begin, the bottom concave mold being placed beneaththe first form (FS1).

The support taking a set of superposed sheets to the bending unit may bethe same support that collects the sheets after bending to take them tothe cooling zone.

Usually, the top form and the bottom bending mold are placed in abending unit taken to the bending temperature.

The superposed sheets in one and the same assembly (or package) havesubstantially the same shape. Their size may vary slightly in thedirection of a diminution of their surface area when moving from thesheet placed in a bottom position to that placed in a top position, sothat the borders of the sheets coincide after the bending.

The invention also relates to a device making it possible to apply themethod according to the invention. The device according to the inventionis a device for bending sets of superposed sheets of glass comprising

-   -   a convex top form FS1 furnished with means of suction around it,        creating an upward airflow blowing over the rim of the sheets,        said suction being sufficient to lift and hold the superposed        sheets against said top form, and    -   a bottom solid concave mold furnished with openings on its main        face and means of suction through these openings, said top form        and said bottom mold having complementary shapes and being able        to move toward one another along a vertical axis to press the        sheets of glass.

In particular, this device may comprise a train of pre-bending supportsmoving to pass one after the other beneath the top convex form (FS1) sothat the sets of superposed sheets may be picked up by said top form(FS1).

This device may furthermore comprise another convex top form (FS2)furnished with means of suction around it, creating an upward airflowblowing over the rim of the superposed sheets, said suction beingsufficient to lift and hold the superposed sheets against it, the bottomconcave mold being able to be moved to pass alternately beneath one andthen the other of the top forms (FS1 and FS2).

For the case in which the device comprises two top forms, it may alsocomprise a train of pre-bending supports moving to pass beneath the twotop convex forms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a top form of the annular frameaccording to an exemplary embodiment of the present disclosure;

FIG. 2 is a perspective view of the bottom mold according to anexemplary embodiment of the present disclosure;

FIGS. 3 a-3 d represent a sequence of a method of the invention usingone top form according to an exemplary embodiment of the presentdisclosure;

FIGS. 4 a-4 d represent a sequence of a method of the invention usingtwo top forms according to an exemplary embodiment of the presentdisclosure;

FIG. 5 depicts a part of the top form coming into contact with theglass.

DETAILED DESCRIPTION

FIG. 1 represents a top form 1 of the annular frame type coming intocontact with the periphery of the top sheet of a set 2 of two superposedsheets. A skirt 3 surrounds the form 1 and is capable of exerting asuction (in the direction of the arrows) sufficient for the twosuperposed sheets to be held against the top form 1. The sucked airblows over the rim 4 of the sheets. Usually, the top form is coated witha material made of refractory fibers (metal and/or ceramic fibers) ofthe felt, woven or knit type, softening the contact of the glass with itand reducing the risks of marking. In addition, the air can circulateinside the fibrous material in all directions which generates anadditional diffusion of the airflow.

FIG. 2 represents a bottom concave mold 5 furnished on its main top facewith a multitude of orifices (openings, holes) 6. A suction createdthrough these orifices bends the superposed sheets by pressing themagainst the main top face of the mold. Usually, this bottom mold iscoated with a material made of refractory fibers (metal and/or ceramicfibers) of the felt, woven or knit type, softening the contact of theglass with it and reducing the risks of marking. In addition, the aircan circulate inside the fibrous material in all directions whichgenerates an additional diffusion of the airflow. This mold has thefinal shape desired for the sheets.

FIG. 3 represents a few sequences of an embodiment of the methodaccording to the invention when it comprises only one top suction form.A train 7 of trolleys 8 each carrying two superposed sheets of glass,takes the sheets (traveling from right to left in FIG. 3) beneath abending unit 9 whose interior is at the bending temperature. Eachtrolley carries a pair of sheets 13 by means of a pre-bending skeleton14. This skeleton may have its contact surface modified in the sense ofan increase in the bends during its path toward the bending unit. Onarrival beneath the unit 9, each trolley has already covered a certaindistance through a tunnel oven 10 so as to cause the pre-bending of thesheets. Beneath the unit 9, the skeleton presents a surface of contact(only for the periphery of the sheets) corresponding to the desiredfinal shape, and the sheets have taken this shape at the periphery butnot in their central region. The unit 9 contains a top form 11 fittedwith a skirt (on the principle of FIG. 1) that is able to movevertically, and a bottom concave mold 12 furnished with openings (on theprinciple of FIG. 2) capable of moving horizontally. The top form is aframe having the shape of a ring and whose shape corresponds to thefinal shape desired for the sheets. The bottom mold has the final shapedesired for the sheets. At the time of FIG. 3 a, the train stops so thata trolley is just at the vertical of the top form. The top form 11 thendescends toward the trolley 8 to pick up the two superposed sheetsthanks to the suction of its skirt (FIG. 3 b). The top form 11 risesagain with the sheets, sufficiently high for the bottom mold 12 to movejust beneath it by horizontal translation. The top form then descendsagain slightly to lightly press the periphery of the sheets against thebottom mold (the skirt is again in operation at that time) as shown inFIG. 3 c. The downward suction through the openings of the main face ofthe bottom mold may then begin to produce the bending by flatteningagainst said face. This suction usually lasts from 1 s to 40 s. When thebending is finished, the top form can rise again with the sheets 13while the bottom mold 12 resumes its rest position on the left. The topform 11 can then descend again to deliver the two bent sheets 13 to thesame trolley 8 as the one that brought it. The train 7 can departleftward so that the next pair of sheets is positioned just beneath thetop form 11 and undergoes the same bending cycle as the pair precedingit. The train 7 travels therefore intermittently, constantly stoppingand starting, each start corresponding to a translation by a distance ofone trolley. The sheets are progressively taken leftward in thedirection of a cooling zone. A fixed horizontal wall 14 placed beneaththe rest position (toward the left) of the bottom mold 12 helps tothermally insulate the bending unit from the tunnel leading to thecooling zone.

FIG. 4 represents a few sequences of an embodiment of the methodaccording to the invention when it comprises two top suction forms FS1and FS2. The start of the method begins like the one explained for FIG.3: the train brings the pairs of sheets pre-bent by gravity onskeletons, and stops so that one of the trolleys (trolley 8 a) carryingthe pair of sheets 13 a stops just beneath the first top form FS1. Thelatter then picks up the pair of sheets 13 a and rises again with itsufficiently high for the bottom concave mold 12 to be able to be placedbeneath the sheets 13 a. The form FS1 then descends again to cause thesheets to be pressed between FS1 and the bottom mold 12. FIG. 4 arepresents this moment in the method. During the pressing, then thesuction exerted on the sheets 13 a, the train moves leftward by adistance of one trolley so that the trolley 8 a is beneath the top formFS2. After the bending begins, the form FS1 rises without lifting thesheets 13 a, the latter remaining placed on the bottom mold 12 whichcontinues the bending by suction. Specifically, even if no top formtouches the sheets at this stage, the sheets remain well pressedtogether without separating because what is important is that a pressingaction takes place at least at the very beginning of the bendingsuction. The bottom mold 12 is then moved leftward (FIG. 4 b) to bebeneath the form FS2. The form FS1 can then descend to pick up the nextpair of sheets 13 b. For its part, the top form FS2 descends to pick upthe pair of sheets 13 a (FIG. 4 c). The form FS1 rises again with thepair 13 b and the bottom mold 12 is then placed beneath the form FS1 topress and bend by suction the pair of sheets 13 b in the same manner ashas been described for the pair of sheets 13 a. The partition 14 whichinsulated the bending unit from the tunnel leading to the cooling zoneis here a movable wall that has been retracted leftward at this stage ofthe method so that the form FS2 carrying the pair of sheets 13 a canrest this pair on the trolley 8 a (FIG. 4 d). Naturally, the support 15actuating the translation of the bottom mold 12 has the necessaryorifice so that the form FS2 can descend to the trolleys. Furthermore itis also possible to actuate the translation of the bottom mold 12 fromthe bottom of the unit, the support 15 then being perpendicular in FIG.4 d. After the pair 13 a has been deposited on the support 8 a, the formFS2 rises again, the movable partition 14 returns leftward beneath theform FS2 in order to insulate the bending unit. The trolley train thenmoves leftward by a distance of one trolley. It can be seen that thisembodiment is more efficient than that of FIG. 3 because it has beenpossible here to move the pair 13 a during its bending and begin toplace in position a new pair 13 b while the bending of the pair 13 a wasfinishing. With this embodiment with two top forms FS1 and FS2, thecycle time-saving corresponds to the trolley transfer time, because itis possible to press while the trolley train is moving.

FIG. 5 represents the part of a top form 16 coming into contact with theglass, said top form being fitted with a skirt 17. The top form 16 isfurnished with a felt element 18 softening the contact with the glass.The skirt 17 is also furnished with a felt element 19 to soften thecontact with the support 20. At the moment represented by FIG. 5, thetop form will pick up two superposed sheets of glass 21 and 22 restingon the support 20. To make the sheets easier to lift, orifices (notshown) may be provided in the support and situated beneath the glass.The sheet 22 is slightly larger than the sheet 21 so that the borders ofthe sheets coincide fully after bending to the final shape. Therepresented part of the skirt 17 may be movable vertically relative tothe form 16. In this manner, when the skirt/top form assembly meets thesheets and the support, it is the skirt 17 that first comes into contactwith the support 20, said skirt then stopping its descent, but the topform continues to descend a little (usually 1 mm to 2 cm) to meet thetop sheet 21. This procedure provides the method with flexibility. Inorder to reduce the necessary flows in the skirt, the distance (d)between the skirt and the top form should be as small as possible. Forexample, this distance may be from 5 mm to 40 mm, for exampleapproximately 20 mm. The rims 23 and 24 of the sheets are situatedbetween the skirt and the top form.

1. A method of bending superposed sheets of glass brought to theirbending temperature, comprising: picking up two superposed sheets with afirst top form by way of a first suction generated by the first topform, the first suction creating an upward airflow blowing over a rim ofthe two superposed sheets and being sufficient to lift and hold the twosuperposed sheets against the first top form; then pressing the twosuperposed sheets between the first top form and a full surface solidconcave bottom form furnished with openings, the pressing the twosuperposed sheets being initiated while the first suction is not yetfinished or is finishing; then forming the two superposed sheets by wayof a second suction of a main face of a bottom sheet of the twosuperposed sheets, the second suction being generated through theopenings of the full surface solid concave bottom form, the secondsuction being initiated before the pressing is finished; then coolingthe sheets, wherein the first top form is a frame.
 2. The method asclaimed in claim 1, wherein the two superposed sheets are pre-bentbefore the picking up the two superposed sheets.
 3. The method asclaimed in claim 1, wherein the two superposed sheets are separated fromthe full surface solid concave bottom form after the forming the twosuperposed sheets by being picked up with aid of a second top form, thesecond top form generating a third suction, the third suction creatingan upward airflow blowing over the rim of the two superposed sheets, thethird suction being sufficient to lift and hold the two superposedsheets against the second top form, the second top form then letting thetwo superposed sheets rest on a support that takes the bent superposedsheets toward a cooling zone for completion of the cooling.
 4. Themethod as claimed in claim 3, wherein the second top form separating thetwo superposed sheets from the full surface solid concave bottom form isdistinct from the first top form picking up the two superposed sheets,the two superposed sheets being moved from a position beneath the firsttop form to a position beneath the second top form by movement of thefull surface solid concave bottom form supporting the two superposedsheets.
 5. The method as claimed in claim 4, wherein when the second topform lets the two superposed sheets rest on the support that takes thetwo superposed sheet to the cooling zone for completion of the cooling,the bending of another set of superposed sheets has already begun, thefull surface solid concave bottom form being placed beneath the firsttop form.
 6. The method as claimed in claim 1, wherein, for the cooling,the two superposed sheets are placed on a cooling support that takes thetwo superposed sheets to a cooling zone, the cooling support being thesame as a support that brought the two superposed sheets under the firsttop form prior to the picking up the two superposed sheets with thefirst top form.
 7. The method as claimed in claim 1, wherein the firstsuction is generated at an outer periphery of the two superposed sheetsso as to lift the two superposed sheets upward and also to suck air outfrom between the two superposed sheets, and the second suction isgenerated by drawing air through the openings of the full surface solidconcave bottom form, the openings of the full surface solid concavebottom form being distributed over a whole surface of the full surfacesolid concave bottom form that faces the main face of the bottom sheetduring the pressing the two superposed sheets.
 8. The method as claimedin claim 7, wherein the first suction is generated through a skirt thatsurrounds the first top form.
 9. The method as claimed in claim 1,wherein the first suction is generated at an outer periphery of the twosuperposed sheets between a peripheral wall of the first top form and askirt disposed outside of the peripheral wall of the first top form. 10.The method as claimed in claim 9, wherein the picking up the twosuperposed sheets with the first top form includes the skirt coming intocontact with a support that carries the two superposed sheets and theperipheral wall of the first top form coming into contact with a topsheet of the two superposed sheets.
 11. The method as claimed in claim10, wherein the picking up the two superposed sheets with the first topform includes the skirt coming into contact with the support thatcarries the two superposed sheets before the peripheral wall of thefirst top form comes into contact with the top sheet of the twosuperposed sheets.
 12. The method as claimed in claim 1, wherein thesecond suction is a vacuum that goes beyond a vacuum pressure of 100mbar.